1. Field of the Invention
The present invention relates generally to an improvement in an ignition circuit for an internal combustion engine, and more particularly to an ignition circuit for an internal combustion engine wherein ignition timing and ignition duty cycle are controlled by electronic circuitry.
2. Description of the Prior Art
Among ignition circuits for internal combustion engines, wherein ignition timing and ignition duty cycle periods are controlled by electronic circuitry, prior art such as that disclosed in the Japanese unexamined published patent application Sho 56-50263 is directed toward two-cylinder engines. Operation of such prior art is described with reference to the graph of FIG. 1, which illustrates intermediate signals for producing ignition signals in accordance with prior art circuitry. These ignition signals are produced by utilizing piston phase detection signals A and B developed by first phase detection sensors (not shown) and second phase detection sensors (not shown) for first and second pistons, respectively. A logic output C is produced by summing the first piston phase detection signal A and the second piston phase detection signal B. By utilizing outputs of an ignition timing computing circuit and an ignition duty cycle computing circuit, desired ignition timing signal D and duty cycle signal E, which are suitably selected for modes of engine operation, are obtainable. Signals D and E are now logically combined to produce a signal F which in turn is combined with a signal G of an output terminal Q of a flip-flop and with a signal H of an output terminal Q of the flip-flop to produce ignition coil signals I and J for first and second engine cylinders, respectively. The resultant signals I and J are used to control electronic switching devices, such as power transistors, which are connected in series with first and second ignition coils to the first and second cylinders, respectively. Primary current waveforms of the first and second ignition coils are as shown in the waveforms K and L, respectively.
In the above-mentioned prior art apparatus, wherein for instance, the internal combustion engine has two cylinders, the start times of currents to the ignition coils are at .theta..sub.d, and the termination times of the currents are at .theta..sub.f. As the speed of revolution of the internal combustion engine increases, the current start and termination times advance, controlled by the ignition timing computing circuit and the ignition duty cycle control circuit. At maximum advance, the start times .theta..sub.d reach the times .theta..sub.1, and the termination times .theta..sub.f coincide with times .theta..sub.2, as shown in FIG. 1.
In such apparatus, the start times .theta..sub.d cannot advance beyond .theta..sub.1 which is on the leading edge of a pulse of signal H, since the times .theta..sub.d are determined by logically processing the signals F and H. In the above-mentioned conventional two cylinder engine ignition apparatus, the maximum advance of the ignition phase is only 180.degree.-(.theta..sub.2 -.theta..sub.3), wherein .theta..sub.2 and .theta..sub.3 are respectively phases of the front and rear edges of the phase detection signals A and B and (.theta..sub.2 -.theta..sub.3) is the required timing advance. In case of a three cylinder engine, the maximum advance of the ignition phase is only 120.degree.-(.theta..sub.2 -.theta..sub.3). In an example wherein (.theta..sub.2 -.theta..sub.3) is 30.degree., the maximum allowable advance of the start time of the ignition pulses for a two cylinder engine is only 150.degree., and this corresponds to about 3.1 m sec for an 8000 rpm engine speed. For a three cylinder engine, the maximum allowable advance of the current start times is only 90.degree., corresponding to about 1.9 m sec for an 8000 rpm engine speed. Therefore, depending on the characteristic of the ignition coil to be used, there is a problem that the duty cycle of the ignition signal may be insufficient. In very high speed revolution of the engine, because of insufficient advance of the start times, the duty cycle of the ignition current pulses fed to the ignition coils becomes too small and accordingly, sufficient spark energy cannot be obtained in the conventional apparatus.
Furthermore, when a vacuum phase advance device or the like is added, the advance becomes even greater than 30.degree., and accordingly the above-mentioned defect becomes even more prominent.